In the past, atrial arrhythmias have been largely undertreated due to the perception that these arrhythmias are relatively benign. As more serious consequences of persistent atrial fibrillation have come to be understood, such as an associated risk of relatively more serious ventricular arrhythmias and stroke, there is a greater interest in providing implantable atrial or dual chamber cardioverter defibrillators for treating atrial arrhythmias.
Atrial fibrillation (AF) is generally treatable with relatively high voltage defibrillation shocks, which are generally painful to the patient, or with less painful high frequency pulse bursts. Atrial flutter (AFL) can be treated by anti-tachycardia pacing therapies, pulse bursts or cardioversion shocks. Reliable discrimination between atrial flutter and atrial fibrillation is important in selecting the appropriate atrial arrhythmia therapy and is also useful in monitoring a patient's arrhythmia disease status, managing medical therapy, and evaluating the effectiveness of arrhythmia therapies.
In atrial flutter, the atria beat at an elevated rate that is highly regular, typically at 200 to 320 beats per minute. The ventricles are unable to respond to each atrial depolarization so a partial block of atrioventricular conduction is usually present causing the ventricles to beat synchronously with every other or every third atrial depolarization. Thus the ventricular heart rate can be in a normal range or elevated during atrial flutter but is typically regular. In atrial fibrillation, the atria depolarize at an elevated rate that is typically irregular. However, atrial fibrillation can occur at regular rates in some patients. Atrial depolarizations are conducted to the ventricles intermittently, causing an irregular ventricular rate. The ventricular rate can be in a normal range or elevated during atrial fibrillation.
Early arrhythmia detection systems for automatic cardioverter/defibrillators relied upon the presence or absence of electrical or mechanical heart activity and/or the rate of the cardiac electrogram to detect ventricular tachycardia or fibrillation. Rate or interval ranges that characterize a tachycardia as opposed to fibrillation could be specified for detecting and discriminating tachycardia and fibrillation. However, tachycardia and fibrillation may have similar or overlapping rates, making it difficult to distinguish high rate tachycardia from fibrillation. Furthermore, since some rapid ventricular rhythms are due to activity that originates in the atria, it has become apparent that evaluation of the ventricular rhythm alone was not adequate in classifying the rhythm or selecting an appropriate arrhythmia therapy.
Methods for improving the specificity of ventricular tachycardia detection and for discriminating ventricular tachycardias from supra-ventricular tachycardias have been employed or proposed for use in commercial dual chamber cardioverter defibrillators. Such methods may include measurements of suddenness of onset, rate variability, waveform morphology, and/or the order and timing of atrial and ventricular events. For example, a method for classifying cardiac arrhythmias by examining the atrial and ventricular activity of the heart is generally disclosed in U.S. Pat. No. 5,107,850 issued to Olive, wherein the regularity and value of the ventricular rate, atrial rate and the atrial-ventricular interval are used for discriminating between atrial rhythms and ventricular rhythms. An arrhythmia detection and classification system that employs a prioritized set of inter-related rules for arrhythmia detection is generally disclosed in U.S. Pat. No. 5,545,186 issued to Olson et al.
Methods for specifically classifying atrial arrhythmias, however, remain dependent on atrial rate information without regard to the ventricular rhythm in current commercially available devices. Such information may include the atrial rate and the regularity of the atrial rate. A range of atrial rates may be specified for detecting atrial flutter and a different, generally higher, range of atrial rates may be specified for detecting atrial fibrillation. However, because the atrial rate could be the same during atrial flutter and atrial fibrillation, specified ranges for atrial flutter and atrial defibrillation detection may overlap and therefore rate information alone is not always adequate for detecting and discriminating atrial flutter and atrial fibrillation. When an atrial rate is detected in this overlap range, atrial cycle length regularity may be used for discriminating between atrial flutter and atrial fibrillation, for example.
Methods for detecting and discriminating atrial fibrillation and atrial flutter using discriminatory signatures of the ventricular cycle lengths displayed in a scatter plot are generally disclosed in U.S. patent application Ser. No. 10/292,285, incorporated herein by reference in its entirety.